1. Field of the Invention
The present invention relates to a torque sensor for sensing a torque applied to a shaft, by utilizing a magnetostriction effect.
2. Description of the Related Art
Various types of torque sensors for sensing a torque applied a shaft (hereinafter, called "shaft torque") are well known. For example, U.S. Pat. No. 4,627,298 (Sahashi et al.) discloses a torque sensor utilizing a magnetostriction effect of a magnetic metallic ribbon having an induced magnetic anisotropy, Japanese Patent Publication (Kokoku) No. 62-28413 discloses a torque sensor employing an amorphous magnetic metallic ribbon as a magnetic metallic ribbon and utilizing a magnetostriction effect of the amorphous magnetic metallic ribbon. In this type of torque sensor utilizing a magnetostriction effect of a magnetic metallic ribbon, a magnetic metallic ribbon having a large magnetostriction effect is fixed on the outer periphery a rotating shaft, and a coil is arranged near the metallic ribbon such that the coil does not contact the shaft. The permeability of the magnetic metallic ribbon, which is caused by a torque applied to the shaft, is changed. A resultant change in impedance of the coil or a change in induced voltage is detected. Based on the detection signal, the shaft torque is sensed. Thus, the torque can be sensed without bringing the sensor into contact with the shaft, and the sensor can be provided on a shaft which is already assembled in an apparatus. In addition, the direction of the torque applied to the shaft can easily be detected by suitably arranging on the shaft two or more magnetic metallic ribbons having induced magnetic anisotropy.
However, this type of torque sensor utilizing the magnetostriction effect of the magnetic metallic ribbon has the following problem. That is, the torque can be accurately sensed in a space free from an external magnetic field, e.g., in a laboratory. However, in a space having a external magnetic field which is variable, the sensitivity of detection varies depending on the external magnetic field. Thus, the torque cannot be sensed exactly. In addition, in the case where the shaft is made of a ferromagnetic body, the shaft is magnetized by the external magnetic field or a magnetic field produced by the coil of the torque sensor, and the magnetization of the shaft deteriorates the sensitivity of detection.